1. Field of the Invention
This invention relates to the mechanical connection between rail vehicles and, in particular, between vehicles for carrying passengers in mass transit applications. This invention more specifically relates to the emergency release of the draft gear or cushioning assembly permitting the vehicles to come together for more controlled absorption of energy and to prevent climbing after collision.
2. Description of Related Art
In railway and transit vehicles, buffing and draft forces between connected vehicles are transmitted to the under frames of the vehicles through drawbars, draft gears and coupler heads (coupler assemblies). The draft gears have cushioning devices which accommodate normally expected forces. Adjacent vehicles are held coupled spaced apart. In the case of abnormal buffing forces which might be encountered on collision, it is desirable to enable the vehicles to come together so that anti-climbers on the ends of the vehicles prevent the end of the trailing vehicle from overriding the lead vehicle. Typically, this function is provided by collapsible draft gears which having release mechanisms based on shear bolts.
As is generally recognized in the railway coupling art, rail transit vehicle coupler assemblies make use of emergency release bolts that break at a designed buff force allowing the draft gear device to telescope into the draft gear housing. The emergency release bolts extend radially through the draft gear housing and into an emergency release ring. The draft gear housing contains an energy-absorbing device that bears against the emergency release ring and emergency release bolts.
The draft gear housing is an integral part of a coupler assembly which is mechanically secured to the underside of its associated vehicle. Coupling and inter-car forces are transmitted from the draft gear assembly to the emergency release ring and to the release bolts by the release ring. Existing coupler assemblies normally employ a rigid emergency release ring suspended in a rigid draft gear housing by radially extending emergency release bolts. A clearance must exist between the rigid emergency release ring and the rigid draft gear housing to permit assembly. The emergency release bolts are designed to shear and break in two pieces when the coupling forces between two vehicles exceed a designed limit as determined by the strength of the emergency release bolts.
Normally, the coupling forces that occur when a consist of rail vehicles is being assembled and connected together for travel with a locomotive or lead vehicle exert less load on the emergency release bolts than the designed limit. The assembly stays intact.
On hard coupling or collision events, forces in excess of the designed limit will be exerted. At this time, the emergency release bolts will break or shear. This allows portions of the draft gear assembly to slide within the draft gear housing and engage a secondary energy dissipation device.
In existing coupler assemblies, the emergency release ring is pulled against the inside wall of the draft gear housing to form a contact at a single location which corresponds to the first emergency release bolt tightened. A gap is formed between the housing and the release ring near the remaining release bolts. This gap allows for forces to repeatedly flex the emergency release bolts in a bending mode. This bending results in reduction in the fatigue strength of the bolts.
The overall structure of the drawbar, draft gear (cushioning unit) and coupler head of one type of mechanical connection for rail vehicles is disclosed in Grau et al. U.S. Pat. No. 6,499,613 entitled “Coupler with Extended Emergency Release and Towing Feature.” This patent discloses primary and secondary release devices, the primary release device being most relevant to this application. The structure of a shear bolt and the need of well-defined shear planes are disclosed Grau et al. U.S. Pat. No. 6,981,599 entitled “High Capacity Shear Mechanism.”